Hardening gelatin-silver halide lithographic offset printing plates



3,091,531 HARDENING GELATiN-SILVER HALIDE LETHG- GRAPHIC OFFSET PTWG PLATES Thomas E. Callear and Merrill W. Killicir, Rochester, N.Y., assignors to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey No Drawing. Filed Feb. 23, 1961, Ser. No. 90,924 8 (Ilaims. (Cl. 96-453) This invention relates to the hardening of lithographic printing plates of certain types to extend the press life thereof.

Lithographic printing plates have a hydrophilic surface which repels lithographic printing ink. In use an image is formed on this hydrophilic surface which image will accept greasy printing ink and enable printing there with by means of a printing press. Lithographic paper printing plates usually include a base coated with a miX- ture of a hardened hydrophilic organic colloid and an inert pigment. Suitable printing plates have a base such as of paper which has been coated such as for example with a mixture of well hardened casein and a pigment which may be clay, barium sulfate, or the like. If the casein coated paper is to be supplied with a layer containing gelatin and pigment, it may have been treated t render the surface less absorptive.

The ink receptive image may be formed photographicmly on a separate sheet and transferred to the printing plate as described in US. Patent No. 2,763,553. In these processes however the transferred image may not be sufficiently durable in the printing operation nor sufficiently ink receptive.

-It has been proposed to apply tannic acid to the press plate for hardening the gelatin in the image transferred thereto.

Frequently however these plates even with tannic acid treatment have a short life on the press because of insufiicient hardening. This may result in eroding away of the image areas causing uneven inking resulting in broken letters on the printed copies. Consequently the plate produces less than the desired number of copies. It has been proposed to employ thiourea with the tannic acid hardener to improve the wearability of the gelatin image of the press plate. Cases have been noted however in which the thiourea has interfered with the development of ink receptivity in the gelatin image.

One object of our invention is to provide an offset printing plate the image of which has been hardened to such an extent that erosion of the image area in the making of copies is reduced to a minimum. Another object of our invention is to provide auxiliary materials for use with tannic acid in obtaining good hardening of the press plate. A still further object of our invention is to provide a tannic acid hardener for the gelatin image of a press plate which gives good hardening of the image without ever adversely affecting the receptivity to greasy printing inks. 7

A still further object of our invention is to provide a composition of tannic acid and a sulfonic acid useful for hardening a gelatin image. Other objects of our invention will appear herein.

These objects are accomplished by transferring to a lithographic printing plate, which will repel greasy printing inks when wet with water, an unhardened gelatin image followed by wetting the surface of the printing plate and the transferred gelatin image with tannic acid and the alkali metal salt of an acid selected from the group consisting of l-naphthol-S-sulfonic acid, 2-naphthol-7-sulfonic acid, and p-phenolsulfonic acid, after which the printing plate is Wet with water and employed in a lithographic printing plate printing press in the customary way.

- percent tannic acid 3,091,531 Patented Mayv 28, 1963 ice The base of a suitable paper printing plate may for example be composed of a melamine-formaldehyde resinsized paper stock of medium Weight coated with a mixture of gelatin and baryta with a gelatin hardening agent such as formaldehyde or aluminum chloride. A suitable printing plate base may also be formed of a sized paper stock of medium weight having a casein coating of about 0.15 to .6 g. of casein per sq. ft. of surface. The casein coating may contain an inert pigment e.g. clay or blanc fixe and may be treated with cuprammonium solution or salt solution such as of zinc acetate before, after, or during application to the casein-pigment coat of a thin coating of a hydrophilic colloid such as sodium alginate, as described in US. Patent No. 2,5 34,650.

The photographic gelatin image which is to be transferred to the printing plate may be formed in various ways but preferably is made and transferred as described in US. Patents Nos. 2,596,756 and 2,716,059 of Yutzy and Yackel. In accordance with these descriptions, a substantially unhardened gelatino silver halide which contains a tanning developing agent or both a tanning and a non-tanning developing agent is exposed to a line or halftone image and developed to form a tanned or hardened gelatin image. The tanned gelatin image may instead be formed by the use of an external developing agent not incorporated in the emulsion layer. After the tanned or hardened gelatin image is thus formed, the remaining gelatin which is unhardened is transferred to an absorbent support such as paper, cloth, or wood, or to the hydrophilic surface of a lithographic printing plate base characterized in that it will repel greasy printing inks when wet with Water.

The unhardened gelatin image thus transferred is soft and hydrophilic. Rendering the image ink receptive and hard and tough so as to withstand abrasion and printing on the printing press, is accomplished by treating the plate after the gelatin image has been transferred to it with a solution of tannic acid and water soluble salt of a. selected sulfonic acid. The sulfonic acids, the alkali metal salts of which have been found to be useful for this purpose, are: l-naphthol-S-sulfonic acid, Z-naphthol- 7-sulfonic acid, and p-phenolsulfonic acid. The hardening may be accomplished by a hardener prepared by combining the sulfonic acid salt into tannic acid solution at the rate of one part of the salt to approximately two parts of tannic acid. However these materials are effective in concentrations varying from 10 to 200 percent based on tannic acid (dry weight). These compounds have no deleterious effects on the tannic acid and do not interfere with the developing of ink receptivity by the gelatin image.

Instead of treating with a mixture of tannic acid and .sulfonic acid salt, the sulfonic acid salt in water solution may be swabbed on to the image area of the press 'plate prior to treatment thereof with tannic acid. All of these salts are sufliciently water soluble to permit easy use in this manner. These hardeners are effective in all proportions but conveniently are employed in concentrations in aqueous solution of 1 percent up to saturation of tannic acid and preferably in proportions of 4 to 10 and 1 part of sulfonic acid salt per 2 parts of tannic acid. When the consecutive treatments with sulfonic acid salt and tannic acid are used, the sulfonic acid salt concentration in water is conveniently within the range of 1 percent up to saturated solution, preferably at 5 percent concentration. The tannic acid concentration used in the two step treatment may be that specified for use in admixture with sulfonic acid salt.

The following examples illustrate our invention.

The image of a typewritten document was formed in a light-sensitive emulsion in the manner described in US. Patent No. 2,716,059 of Yutzy and Yackel and the unhardened gelatin of the sensitive material was transferred to a lithographic printing plate. A printing plate of the type described above was used. The transferred gelatin image was swabbed for 20 seconds with a solution of .napthol-7-sulfonic acid each at a concentration of percent of tannic acid and where applicable 4 percent of sulfonic acid salt. The plates thus treated were then heated for 20 seconds with light from two photoflood lamps at about 1 foot distance and were then moistened with water, placed on an offset press and run under standard printing conditions. The plate which was hardened with tannic acid only gave no more than 50 copies, whereas the plate hardened with tannic acid and p-phe nolsulfonic acid salt gave 200 copies, that hardened with tannic acid and l-naphthol-S-sulfonic acid salt 250 copies, and that hardened with tannic acid and 2-naphthol-7-sulfonic acid salt 150 to 200 copies.

'In another similar run but using medium and long run printing plates, the plates hardened with tannic acid only gave but 300 copies using medium run plates and 800 copies using long run plates. The plates hardened with a mixture of tannic acid and salts of the sulfonic acids however gave the following numbers of copies on the press:

N o. of Copies Hardener Medium Long Run Run Tannie acid+l-Naphthol-8-sulfonic acid 1 1, 000 1, 800 Tannic acid-l-Z-Naphthol-7-sulfonic acid 1 1, 000 1, 800 Tanm'c acid+p-Phenolsullonic acid 800 1,200

1 In the form of its sodium salt.

We claim:

1. The method of preparing a lithographic printing plate having good lithographic ink receptivity and durability, which comprises forming by exposure and development in a gelatino silver-halide emulsion layer of unhardened gelatin a, hardened gelatin image, transferring the unhardened gelatin portions of said emulsion layer to a lithographic printing plate which will repel greasy printing inks when wet with water, wetting the surface of the printing plate and transferred gelatin image with a solution containing approximately 4-10% of tannic acid and approximately 10-20%, based on the weight of the tannic acid, of a sulfonic acid salt selected from the group consisting of the water-soluble salts of l-naphtho'l- 8-sultonic acid, 2-naphthol-7-sulfonic acid and p-phenolsulfonic acid, and then wetting the surface of the printing plate with water and placing it in an olfset printing press.

2. The method of preparing a lithographic printing plate having good lithographic ink receptivity and durability which comprises forming by exposure and development in a gelatino silver-halide emulsion layer of unhardened gelatin a hardened gelatin image, transferring the unhardened gelatin portions of said emulsion layer to a lithographic printing plate which will repel greasy printing inks when wet with water, wetting the surface of the printing plate and transferred gelatin image with a solution containing approximately 4-10% of tannic acid and approximately 10-20%, based on the weight of the tannic acid, of the sodium salt of 1-naphthol-8-sulfonic acid and then wetting the surface of the printing plate with water and placing it in an offset printing press.

3. The method of preparing a lithographic printing plate having good lithographic ink receptivity and durability which comprises forming by exposure and development in a gelatino silver-halide emulsion layer of unhardened gelatin a hardened gelatin image, transferring the unhardened gelatin portions of said emulsion layer to a lithographic printing plate which will repel greasy printing inks when wet with water, wetting the surface of the printing plate and transferred gelatin image with a solution containing approximately 1-10% of tannic acid and approximately 10-20%, based on the Weight of the tannic acid, of 2-naphthol-7-sulfonic acid in the form of its alkali metal salt, and then wetting the surface of the printing plate with water and placing it in an offset printing press.

4. The method of preparing a lithographic printing plate having good lithographic ink receptivity and durability which comprises forming by exposure and development in a gelatino silver-halide emulsion layer of unhardened gelatin a hardened gelatin image, transferring the unhardened gelatin portions of said emulsion layer to a lithographic printing plate which will repel greasy printing inks when Wet with water, wetting the surface of the printing plate and transferred gelatin image with a solution containing approximately 4-10% of tannic acid and approximately 10-20%, based on the weight of the tannic acid, of p-phenolsulfonic acid in the form of its a1- kali metal salt, and then wetting the surface of the printing plate with water and placing it in an offset printing press.

5. An aqueous solution for treating a lithographic printing plate having a gelatin image thereon comprising approximately 4-10% of tannic acid and approximately 10-20%, based on the weight of the tannic acid, of a sulfonic acid salt selected from the group consisting of the water soluble salts of l-naphthol-S-sulfonic acid, 2-naphthol-7-sulfonic acid, and p-phenolsulfonic acid.

6. An aqueous solution for treating a lithographic printing plate having a gelatin image thereon comprising approximately 4-10% of tannic acid and approximately 10-20%, based on the weigh-t of the tannic acid, of the sodium salt of 1-naphthol-8-sulfonic acid.

7. An aqueous solution for treating a lithographic printing plate having a gelatin image thereon comprising approximately 4-10% of tannic acid and approximately 10-20%, based on the weight of the tannic acid, of 2- naphthol-7-sulfonic acid.

8. An aqueous solution for treating a lithographic printing plate having a gelatin image thereon comprising approximately 4-10% of tannic acid and approximately 10-20%, based on the weight of the tannic acid, of p-phenolsnlfonic acid.

References Cited in the file of this patent UNITED STATES PATENTS 1,901,536 Schafer Mar. 14, 1933 1,908,916 Noerr et al May 16, 1933 2,184,289 Dangelmajer Dec. 26, 1939 2,716,059 Yutzy et a1. Aug. 23, 1955 2,794,388 Lake et a1. June 4. 1957 

1. THE METHOD OF PREPARING A LITHOGRAPHIC PRINTING PLATE HAVING GOOD LITOGRAPHIC INK RECEPTIVITY AND DURABILITY, WHICH COMPRISES FORMING BY EXPOSURE AND DEVELOPMENT IN A GELANTINO SILVER-HALIDE EMULSION LAYER OF UNHARDENED GELATIN A HARDENED GELATIN IMAGE, TRANSFERRING THE UNHARDENED GELATIN PORTIONS OF SAID EMULSION LAYER TO A LITHOGRAHIC PRINTING PLATE WHICH WILL REPEL GREASY PRINTING INKS WHEN WET WITH WATER, WETTING THE SURFACE OF THE PRINTING PLATE AND TRANSFERRED GELATIN IMAGE WITH A SOLUTION CONTAINING APPROXIMATELY 4-10% OF TANNIC ACID AND APPROXIMATELY 10-20%, BASED ON THE WEIGHT OF THE TANNIC ACID, OF A SULFONIC ACID SALT SELECTED FROM THE GROUP CONSISTING OF THE WATER-SOLUBLE SALTS OF 1-NAPHTHOL8-SULFONIC ACID, 2-NAPHTHOL-7-SULFONIC ACID AND P-PENOLSULFONIC ACID, AND THEN WETTING THE SURFACE OF THE PRINTING PLATE WITH WATER AND PLACING IT IN AN OFFSET PRINTING PRESS. 